1. Field of the Invention
The present invention relates to a refrigerant overcharge prevention system for precisely and easily detecting the finish of a proper refrigerant charge when refrigerant is charged into a refrigerant circuit of a cooling apparatus using a refrigerant charging apparatus.
2. Description of the Prior Art
Typical conventional cooling apparatus and refrigerant charging apparatus are constituted, for example, as shown in FIG. 11. The cooling apparatus has refrigerant circuit 1 formed from pipe 7. A refrigerant such as freon gas is circulated in refrigerant circuit 1. Compressor 2, condenser 3, receiver dryer 4, expansion valve 5 and evaporator 6 are provided in refrigerant circuit 1 sequentially in the circulation direction of the refrigerant which is shown by arrows. Refrigerant charging apparatus 8 comprises bomb 9 for storing refrigerant therein, gage manifold 10, having pressure gages 11, connected to the bomb via pipe 12 having valve 13, and refrigerant supply pipes 14, having valves 15, connected to refrigerant circuit 1.
In the refrigerant charging apparatus 8, the pressure of refrigerant to be supplied is controlled by valves 13 and 15 while the pressure is observed with pressure gages 11. In such a conventional refrigerant charging system, however, there are the following problems.
When refrigerant is charged, an operator terminates charging after recognizing by observation through a sight glass that bubbles of refrigerant (refrigerant in vapor phase) present in the liquid line of refrigerant circuit 1 or in receiver dryer 4 disappear. Namely, since the bubbles of refrigerant disappear when the amount of refrigerant present in refrigerant circuit 1 reaches a required amount, completion of the charge can be recognized by the state with no bubbles. However, if the rate of refrigerant charge is too fast, the bubbles of refrigerant do not disappear immediately after the amount of refrigerant present in refrigerant circuit 1 has reached a required amount. If the refrigerant charge is continued thereafter, an excessive amount of refrigerant is charged. It is difficult to prevent such an overcharge of refrigerant with the conventional system.
FIG. 9 shows the relationship between the amount of refrigerant charged into the refrigerant circuit and the pressure in the high-pressure side path of the refrigerant circuit. Although the pressure gradually increases as the amount of charged refrigerant increases, there exists an interval (A to B) in which the pressure is almost constant. This interval (A to B) constitutes an interval of proper refrigerant charge amounts. If refrigerant is further charged after the charged amount reaches point "B", the pressure again increases. If the charged amount exceeds point "C" which constitutes the upper limit of the charged amount for circuit 1, the pressure raises rapidly. Thus, the hatched area in the graph of FIG. 9 constitutes an overcharge area wherein the charged amount may cause malfunction of or damage to the cooling apparatus (for example point "D").
FIG. 10 shows the relationship between the refrigerant charge time and the amount of refrigerant charged into the refrigerant circuit. The marks "o" on the ends of the respective characteristic lines indicate points at which the bubbles of refrigerant disappear. As shown in FIG. 10, if the rate of refrigerant charge is too fast, the bubbles of refrigerant do not disappear until the charged amount enters into the overcharge area depicted by the hatch lines (for example, point "D"). Point "D" shown in FIG. 10 corresponds to the point "D" shown in FIG. 9. The interval of proper charge amounts is depicted by "R".
Thus, in the conventional system, since the time when the amount of charged refrigerant reaches a proper amount often differs from the time when the bubbles of refrigerant disappear, it is difficult to charge precisely a proper amount of refrigerant. Moreover, it is difficult to charge consistently refrigerant at an adequate rate in order to prevent inconsistency in refrigerant charge times.